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Ag superconducting composites
PflYSICA[
Physica B 194-196 (1994) 2283-2284 North-Holland
Microstructure and transport property in silver doped BiPbSrCaCuO(2223)/Ag superconducting composites Y. C. Guo, H. IC Liu and S. X. Dou School of Materials Science and Engineering. The University of New South Wales, P.O. Box 1, Kensington, NSW 2033, Australia Silver-sheathed (Bi,Pb)2SqCa2Cu3Ol0 composite tapes have been doped with varying silver levels and their microstructures and transport properties have been investigated. X-ray diffraction and resistivity measurements indicate that the silver doping causes neither change in the value of Tc nor decomposition of high-T¢ phase. However, the electrical measurements show that the silver does influence the critical current density (J~), which decreases with increasing silver dopant content when tapes are annealed with same temperature. The microstructural analyses re.veal that silver exists as an isolated phase inside the tape without visible reaction and diffusion with superconductor matrix, but the undesirable morphology of doped silver particles cause a degradation of grain alignment. Investigation of the effect of silver doping on the property of Bi-based superconductors is of significant technical importance because silver seems to be the most suitable metal used in fabricating superconductor-metal composites such as wires, tapes, ribbons and films. Unfortunately, so far the published results on this aspect are very inconsistent. Conflicting re,suits have bi~n reported on the influence of silver doping on the high-T~ phase formation, critical temperature O'~), and J~ of Bi-Pb-Sr-Ca-Cu-O superconductor [1,2,3]. Therefore, more work is needed to clarify the behaviour of silver in the B i - b a s e d superconductors. In this paper, we report the results of our investigation of the influence of silver doping on the microslructure and superconducting property of BPSCCO(2223)/Ag composite tapes. Powders were prepared by t h e r m a l decomposition of metal nitrates solutions having a cation ratio of Bi/Pb/Sr/Ca/Cu=l.8/0.4/2.0/2.2/3.0. The dried powders were calcined at 800°C for 2h, mixed with appropriate amount of fine AgzO powders according a formula (Bi,PbhSraCa2Cu~Ag~Oa0 (x=0.0-3.5), pressed into pellets, and sintered at 840°C for 15h twice with a intermediate grinding. The powders were then packed into silver tubes and cold-worked into thin wires and tapes. The resultant tapes were subjected to a thermomechanical process consisting of several cycles of pressing and sintexing. All the
sintering was conducted at 830-8400C for a period up to lOOh for each step in air. T© and J~ were measured by the standard four-point probe technique. X-ray diffraction (XRD) analyses and scanning electron microscopy (SEM) observation were also performed to check the phase assemblage and microstructure of samples. •
:(2223)
•
• X=&5 # Ag
•
-iii
o
tb
F~.I: ~
,'o
*
.
3'o go 5'0 AN~LE (ZO)
6'o
¢o
patterns for silver doped tapes
Figure 1 shows the XRD patterns for a set of samples doped with various silver contents (x=O.O, 0.5, 3.5) after sintering at 832°C for 380h. The major peaks can be indexed with the high-T c (2223) phase pattern and some small peaks are corresponding to the low-To (2212) phase and
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1774-G
.
2284 silver peaks in silver doped samples. The (001) peaks are sharp and strong, indicating that the grains inside the tape are highly orientated with their c-axis perpendicular to the wide surface of tapes. The volume fraction of high-T o phase estimated from the peak intensities (except silver peaks) is high (~.95%) and almost identical for all samples independent of silver dopant contents, suggesting that silver doping did not affect the high-To phase formation. The inert behaviour of silver doping to the superconductor was also found in the T, measurements. The resistivity for all samples doped with various silver contents (x=0.0-3.5) shown a sharp drop at about l l 0 K and reached zero nearly at an identical temperature of 107K, indicating the silver had no influence on the To of samples. This is in good agreement with the results of above phase analyses, but in contrast to the results reported previously[4].
3,
o
Fig.3: SEM micrographs for tapes with silver content of (a) X=0.0 and Co) X=1.6
~2 v
0
%t
b-t
Silver Doped Bi(2223)/Ag Tapes
0
o.o
o.'5 ,.'o
,.'5
,.'o
2.'5
SILVER CONTENT (X)
,.o
Fig.2: Maximum Jo vs. silver dopant content However, the silver doping does show a influence on the Jc of the Bi-Pb-Sr-Ca--Cu-O tapes. Figure 2 presents the Jo values versus silver content (x) for tapes annealed at 832°C for 300h. The Jo values were corrected by considering the volume of doped silver particles in the core of tapes. As can be seen, the silver doping shows a detrimental effect to the J, of sample. The observed electrical properties can be explained by the results of microstruetural analyses. Shown in figure 3 are the SEM micrographs of a undoped sample (fig.3a) and a silver doped sample (fig.3b). It is noted that silver seems to be present as isolated particles in the BiPh-Sr-Ca-Cu-O matrix with no detectable mutual solubility between them within the resolution of the
EDS analysis. The silver did not enter the lattice of superconductor, so has no influence to the high-T¢ phase formation and T, value of samples. However, compared to the superconductor grains which has a extremely large ratio of the transverse dimensions to the thickness the doped silver particles are thick and irregular in shape. The undesirably shaped silver particles cause a angle between the neighbouring superconductor grains, resulting in a degradation of grain alignment. And this degradation becomes more pronounced when silver content increases. It is well known that high degree of grain ali~,nment is one of the essential factor for the superconductor to carry a large current[5]. The degradation of grain alignment may be the main reason for the Jc decrease in the silver doped tapes. REFERENCES:
1: 2: 3: 4: 5:
S. Jin etal, Appl. Phys. Lett., 52 (1988) 1628 A. Oota et al, J. Appl. Phys., 71 (1992) 5997 Y. Ishida etal, Physica C, 190 (1991) 67 S. Dou et al, AppL Phys. Lett., 56 (1990) 493 S. Jin etal, Mater. Sci. Eng., B7 (1992) 243
Physica B 194-196 (1994) 2283-2284 North-Holland
Microstructure and transport property in silver doped BiPbSrCaCuO(2223)/Ag superconducting composites Y. C. Guo, H. IC Liu and S. X. Dou School of Materials Science and Engineering. The University of New South Wales, P.O. Box 1, Kensington, NSW 2033, Australia Silver-sheathed (Bi,Pb)2SqCa2Cu3Ol0 composite tapes have been doped with varying silver levels and their microstructures and transport properties have been investigated. X-ray diffraction and resistivity measurements indicate that the silver doping causes neither change in the value of Tc nor decomposition of high-T¢ phase. However, the electrical measurements show that the silver does influence the critical current density (J~), which decreases with increasing silver dopant content when tapes are annealed with same temperature. The microstructural analyses re.veal that silver exists as an isolated phase inside the tape without visible reaction and diffusion with superconductor matrix, but the undesirable morphology of doped silver particles cause a degradation of grain alignment. Investigation of the effect of silver doping on the property of Bi-based superconductors is of significant technical importance because silver seems to be the most suitable metal used in fabricating superconductor-metal composites such as wires, tapes, ribbons and films. Unfortunately, so far the published results on this aspect are very inconsistent. Conflicting re,suits have bi~n reported on the influence of silver doping on the high-T~ phase formation, critical temperature O'~), and J~ of Bi-Pb-Sr-Ca-Cu-O superconductor [1,2,3]. Therefore, more work is needed to clarify the behaviour of silver in the B i - b a s e d superconductors. In this paper, we report the results of our investigation of the influence of silver doping on the microslructure and superconducting property of BPSCCO(2223)/Ag composite tapes. Powders were prepared by t h e r m a l decomposition of metal nitrates solutions having a cation ratio of Bi/Pb/Sr/Ca/Cu=l.8/0.4/2.0/2.2/3.0. The dried powders were calcined at 800°C for 2h, mixed with appropriate amount of fine AgzO powders according a formula (Bi,PbhSraCa2Cu~Ag~Oa0 (x=0.0-3.5), pressed into pellets, and sintered at 840°C for 15h twice with a intermediate grinding. The powders were then packed into silver tubes and cold-worked into thin wires and tapes. The resultant tapes were subjected to a thermomechanical process consisting of several cycles of pressing and sintexing. All the
sintering was conducted at 830-8400C for a period up to lOOh for each step in air. T© and J~ were measured by the standard four-point probe technique. X-ray diffraction (XRD) analyses and scanning electron microscopy (SEM) observation were also performed to check the phase assemblage and microstructure of samples. •
:(2223)
•
• X=&5 # Ag
•
-iii
o
tb
F~.I: ~
,'o
*
.
3'o go 5'0 AN~LE (ZO)
6'o
¢o
patterns for silver doped tapes
Figure 1 shows the XRD patterns for a set of samples doped with various silver contents (x=O.O, 0.5, 3.5) after sintering at 832°C for 380h. The major peaks can be indexed with the high-T c (2223) phase pattern and some small peaks are corresponding to the low-To (2212) phase and
0921-4526/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved S S D I 0921-4526(93)1774-G
.
2284 silver peaks in silver doped samples. The (001) peaks are sharp and strong, indicating that the grains inside the tape are highly orientated with their c-axis perpendicular to the wide surface of tapes. The volume fraction of high-T o phase estimated from the peak intensities (except silver peaks) is high (~.95%) and almost identical for all samples independent of silver dopant contents, suggesting that silver doping did not affect the high-To phase formation. The inert behaviour of silver doping to the superconductor was also found in the T, measurements. The resistivity for all samples doped with various silver contents (x=0.0-3.5) shown a sharp drop at about l l 0 K and reached zero nearly at an identical temperature of 107K, indicating the silver had no influence on the To of samples. This is in good agreement with the results of above phase analyses, but in contrast to the results reported previously[4].
3,
o
Fig.3: SEM micrographs for tapes with silver content of (a) X=0.0 and Co) X=1.6
~2 v
0
%t
b-t
Silver Doped Bi(2223)/Ag Tapes
0
o.o
o.'5 ,.'o
,.'5
,.'o
2.'5
SILVER CONTENT (X)
,.o
Fig.2: Maximum Jo vs. silver dopant content However, the silver doping does show a influence on the Jc of the Bi-Pb-Sr-Ca--Cu-O tapes. Figure 2 presents the Jo values versus silver content (x) for tapes annealed at 832°C for 300h. The Jo values were corrected by considering the volume of doped silver particles in the core of tapes. As can be seen, the silver doping shows a detrimental effect to the J, of sample. The observed electrical properties can be explained by the results of microstruetural analyses. Shown in figure 3 are the SEM micrographs of a undoped sample (fig.3a) and a silver doped sample (fig.3b). It is noted that silver seems to be present as isolated particles in the BiPh-Sr-Ca-Cu-O matrix with no detectable mutual solubility between them within the resolution of the
EDS analysis. The silver did not enter the lattice of superconductor, so has no influence to the high-T¢ phase formation and T, value of samples. However, compared to the superconductor grains which has a extremely large ratio of the transverse dimensions to the thickness the doped silver particles are thick and irregular in shape. The undesirably shaped silver particles cause a angle between the neighbouring superconductor grains, resulting in a degradation of grain alignment. And this degradation becomes more pronounced when silver content increases. It is well known that high degree of grain ali~,nment is one of the essential factor for the superconductor to carry a large current[5]. The degradation of grain alignment may be the main reason for the Jc decrease in the silver doped tapes. REFERENCES:
1: 2: 3: 4: 5:
S. Jin etal, Appl. Phys. Lett., 52 (1988) 1628 A. Oota et al, J. Appl. Phys., 71 (1992) 5997 Y. Ishida etal, Physica C, 190 (1991) 67 S. Dou et al, AppL Phys. Lett., 56 (1990) 493 S. Jin etal, Mater. Sci. Eng., B7 (1992) 243